N-butane/isobutane fractionation

ABSTRACT

An n-butane/isobutane splitter is operated by compressing the isobutane overhead to increase its condensing temperature, using the compressed overhead to heat bottoms in a reboiler, which is operated to condense the overhead and cooling the condensed overhead to a temperature no lower than the temperature on the top tray of the splitter and no higher than 20° F. above the temperature on the top tray, whereby the throughput of the splitter is increased by 10 to 20%.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for the separation of normalbutane and isobutane by fractionation. More particularly the inventionrelates to the operation of an n-butane/isobutane splitter employingcompressed overheads to heat reboiler bottoms to provide a portion ofthe energy used for the fractionation.

2. Related Art

The use of heat exchange (usually indirect) of one stream to heatanother is widely practiced. For example in U.S. Pat. No. 2,619,814(Kniel) the reboiler may be heated by compressed overheads, in U.S. Pat.No. 3,509,728 (Mercer) compressed overheads heated an evaporator and inU.S. Pat. No. 4,022,597 (Bacon) overheads and bottoms are used to heatincoming feed.

In the last several years n-butane/isobutane splitters, i.e.,fractionation units used to separate n-butane from isobutane, haveemployed heat pumps to increase the pressure of the overhead vapors,thereby increasing the condensing temperature of the vapors, which arethen heat exchanged with bottoms from the column in a reboiler torecover some of the heat and use it to drive the distillation, e.g.,U.S. Pat. No. 4,559,108 to Ahlberg discloses a C₄ fractionation whereinthe overhead is split, a portion is compressed, used to heat thebottoms, then recombined with the uncompressed portion, cooled andcollected for product recovery and/or reflux.

Similar types of procedures have been described for various hydrocarbonseparations, e.g., U.S. Pat. No. 4,230,535 to Howard discloses that twoclose boiling chemicals may be separated by removing an overhead vaporstream, condensing a portion of the vapor, heating the vapor stream thencompressing the heated vapors and supplying heat to a reboiler bycondensing a portion of the compressed vapor stream; U.S. Pat. No.3,002,358 to Dierl discloses a propylene distillation wherein a portionof the overhead is compressed and used to heat two reboilers and aportion returned to the column as reflux; and U.S. Pat. No. 4,277,268 toSpangler, Jr. discloses a fractionator split into a stripping sectionand a rectifying section wherein the overhead vapors from the rectifyingsection are heat exchanged with the bottoms of the stripping section.

For economic reasons the system of using the increase in condensationtemperature which results from compression to heat the bottoms andprovide boil up can only be employed in those systems where the bottomstemperature and the overhead temperature are not widely different suchas n-butane/isobutane and propane/propylene separations.

Operation of a conventional n-butane/isobutane splitter may employ aheat pump to compress a portion of the overheads to thereby raise thecondensing temperature of the overheads which are then heat exchanged ina reboiler with a portion of the bottoms to provide heat for the boil upin the column. The contact in the reboiler condenses the overheads whichare accumulated, with a portion being yielded to isobutane product and aportion being returned as reflux to the splitter (fractionating column)Reflux is returned to the column as a mixture of vapor and liquid,usually on to or above the first tray. Even though the temperaturegradient along the splitter is generally less than 35° F., the pressureof the overheads needs to be 50-100 psi or more, to provide a sufficienttemperature differential in the reboiler (heat exchanger) to maintainthe temperature in the bottom of the column for boil up and to drive thefractionation.

In the prior operation the temperature of the reflux is such that around17% of the reflux was vaporized on the top tray, which increased thevapor from the top tray by that amount and increased the flow throughthe compressor, correspondingly.

In the present invention it was found that reducing the temperature ofthe reflux to a temperature no lower than the temperature on the toptray and no higher than about 20° F. greater than the temperature on thetop tray, reduces the amount of reflux vaporized and allows the columnto be operated at higher throughputs, hence increasing the columns'efficiency. Reflux returned to the tower is less likely to vaporize ifthe temperature is closer to the temperature on the top tray but nolower than temperature on the top tray.

SUMMARY OF THE INVENTION

Briefly, the present invention is a fractional distillation for theseparation of isobutane from normal butane in which the overhead iscompressed to increase its condensing temperature with the compressedoverhead then being used to heat the bottoms to provide the boil upwhile the overheads are condensed. The compressed overheads arecondensed in the reboiler by heat exchange with a portion of thebottoms, then a portion of the condensed overheads are subsequentlycooled and returned to the fractional distillation as reflux. Thetemperature of the reflux must be at least as high as the top of thetower, i.e., the temperature on the first tray, in order to avoidovercondensing vapor coming to the top tray, e.g., flooding, but nohigher than about 20° F. greater than the temperature at the top of thetower, preferably no higher than 15° F. greater than the temperature atthe top of the tower. The higher the temperature of the reflux (abovethe temperature of the top tray) the more it is volatilized with anundesirable increase in the vapors in the overhead, which in effectreduces the throughput capacity of any given unit. Economicconsiderations play a part in adapting any system to the presentinvention. Hence the amount of cooling which can be justifiedeconomically (i.e., energy expense) must be balanced with the benefit ofincreased throughput (i.e., energy saving). Thus, although a temperatureas close to and slightly above that at the top of the column is mostdesirable, realistically a temperature within 20° F. of that temperatureprovides substantial benefits since uncooled reflux will be well inexcess of this, e.g. 30° F. or greater than the temperature of the toptray. Generally an n-butane/isobutane splitter is operated at 50 to 100psig with corresponding overhead temperature of 90° to 103° F. andbottoms temperature of 123° to 140° F.

After the overhead is condensed it is accumulated and a portion then isdirected back to the column as reflux (the remainder is isobutaneproduct). The condensed overhead is usually well above the temperatureat the top of the tower at this point, e.g., 30° F. or greater. Feedingthis material back as reflux can result in 10 to 20% less capacity for agiven n-butane/isobutane splitter.

More specifically, the present invention is a process for separatingisobutane from n-butane by fractionation comprising feeding ahydrocarbon stream consisting essentially of isobutane and n-butane to afractionation zone (i.e., column) preferably having therein a pluralityof trays. Heat is supplied to said fractionation zone to vaporize saidisobutane and to provide an overhead stream consisting essentially ofisobutane, vapor. The overhead vapor stream is compressed to increasethe condensing temperature thereof to a temperature sufficiently greaterthan the temperature of the bottoms of said fractionation zone(generally 10° to 20° F. increase in the condensing temperature) wherebyindirect contact of said compressed overheads with a portion of thebottoms from said fractionation zone increases the temperature of thebottoms portion to (1) vaporize said portion of the bottoms, (2) provideboil up in said fractionation zone, and (3) condense said compressedoverhead. The condensed overhead are recovered and a portion of thecondensed overhead are cooled to a temperature no less than thetemperature at the top of the column, preferably no less than thetemperature on the top tray if trays are present and no greater than 20°F. above the temperature at the top of the column. The portion of cooledcondensed overhead is returned to the top of the column as reflux.

The indirect contact in the reboiler is preferably achieved bymaintaining a liquid level of condensed overheads in areboiler/condenser, said liquid level being adjusted to extract amaximum amount of heat from the compressed overheads to give the lowesttemperature leaving the reboiler

When the term "tray" is used herein, it includes all of the varioustypes of internal structures used for intimately contacting gases andliquid, such as tray columns, including cross-flow plate and counterflowplate types, bubble cap columns, inert packing materials and the like.In these conventional systems, the reflux is returned to the top of thecolumn and usually on to the top tray, plate, cap or the like.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a diagrammable flow sheet of the process of the presentinvention.

PREFERRED EMBODIMENT AND DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 provides a flow diagram of the present process. The feed,predominantly a C₄ stream of n-butane and isobutane enters thefractionation column 10 via line 12. The column preferably hasconventional sieve trays, preferably about 80 trays. The feed enters thecolumn in the upper middle quadrant, although it could be fed atdifferent points, just as other types and numbers of trays can be used.

The heat for the fractionation is supplied by compressing the overheadvapors in compressor 18 then using the compressed overhead vapors toheat the bottoms circulating from column 10 through the reboiler 22.When heated to the appropriate temperature for the pressure conditionsin the column, the lower boiling isobutane (and lighter hydrocarbons)will vaporize and ascend upward through the column while the heaviermaterials, n-butane (any C₅ or heavier hydrocarbons) will descendthrough the column to form the bottoms which are removed via line 16.The vapor rising will tend to concentrate the isobutane, such that bythe time it reaches the top of the column it has concentrated to thepoint where it is high purity isobutane and a high purity n-butanebottoms.

Since both n-butane and isobutane are gases at atmospheric pressure, theuse of a pressured system is essential to being able to carry out afractional distillation. The column may be operated at pressures between50 and 100 psig, preferably 60 to 80 psig. The heat pump system usingcompressed overheads to supply heat for the reboiler only works wherethe temperature differential between the overhead and the bottoms isclose, as it is in the case of n-butane and isobutane. The temperaturedifferential (overheads to bottoms) in a commercial n-butane/isobutanesplitter is about 23° to 35° F.

Thus by heating the mixed n-butane and isobutane under pressure in thecolumn as described the overhead 14 is recovered as high purityisobutane. The overhead is compressed in compressor 18 whereby energy inthe form of heat is added to the vaporous overhead by mechanical work(compression). The compressed isobutane overhead 20 is now at a pressuresufficiently high to give a condensing temperature to heat the reboiler22, that is, to provide the heat source to heat the liquid in thereboiler sufficiently to provide all of the heat (boil up) needed tosustain the fractionation. Even though energy is required to operate thecompressor it is less than that required to operate a conventionaldistillation without compression.

When the compressed overheads indirectly contact the bottoms fractionthe overheads are condensed and the condensed overheads sent via 24 toaccumulator 26 where a portion is yielded as isobutane product 34 and aportion 28 sent to cooler 30.

According to a preferred embodiment the liquid level and the rate ofcondensed overheads flow in the reboiler 22 are adjusted to permit thelowest temperature of the condensed overheads leaving the reboiler,while maintaining the condensing capacity of the reboiler at itsmaximum. Precise control of the condensate level can maximize the degreeof condensation being obtained while also obtaining the lowesttemperature of the condensate leaving the reboiler.

From the compressor 18 through cooler 30 the pressure in the system issubstantially greater than the pressure in the fractionation column.Hence, an expansion valve 38 (or other device for this purpose) isprovided to allow for the cooled reflux stream 32 to adjust to thepressure of the column. In prior operation where the reflux had not beencooled as taught herein, the reduction of the pressure at the expansionvalve resulted in a substantial vaporization of the reflux even beforeit was returned to the tower. This vapor combined with the flashvaporization of the overheated liquid portion of the reflux results in asubstantial incidental overhead vapor burden, which when reducedaccording to the present invention increases the capacity of the sameequipment.

The reflux 32 is returned to the top of column which in this embodimentis on the top tray 36. An isobutane product 34 is yielded fromaccumulator 26.

Such conventional items as pumps, valves, drains and the like areomitted from the flow diagram but their location and functioning isobvious to those in the art. In a conventional n-butane/isobutanesplitter the volume of overheads is in excess of that required forheating the bottoms thus a trim condenser was employed to condense thisexcess, however, the system according to the present invention does notrequire a trim condenser.

Current fractionating skill allows very narrow and precise cuts to betaken. Hence the feed to the n-butane/isobutane splitter ispredominantly C₄ with less than 5 vol. % total heavier and/or lighterhydrocarbons. The present invention is designated for the separation ofa C₄ stream containing isobutane and normal butane as its principalcomponents. Generally, a feed suitable for this procedure would contain5 to 95 mole % isobutane, 5 to 95 mole % n-butane, 0 to 20 mole %butenes, 0 to 5 mole % C₃ 's and lighter, and 0 to 5 mole % C₅ 's andheavier. Preferably, the feed will contain 20 to 80 mole % isobutane and20 to 80 mole % n-butane with less than 1 to 3 mole % C₃ 's and 1 to 3mole % C₅ 's and heavier. One source of suitable feed to the splitter 10is the overheads from a debutanizer.

In order to demonstrate the improvement obtained according to thepresent invention a comparison between the splitter operated with refluxcooled in accordance with the invention and in the prior manner ispresented. In the TABLE the two operations are compared at variouspoints along the flows. The feed used in these examples contained 36.8mole % isobutane, 61.6 mole % n-butane, 1.2 mole % propane and 0.4 mole% isopentane. For this illustration the overhead flow is constant forboth operations (compressor capacity) and the temperature in theaccumulator is the same. The distinction in the two operations can beseen in the Table wherein the vaporized reflux, according to the presentinvention is only 99,000 lbs./hr vapor compared to 204,000 lbs./hr vaporreflux in the prior operation. This coupled with the reduction in theflashing of the cooler liquid portion of the reflux, allows anadditional 29,546 pounds per hour of feed to the column at the sameoverhead flow rates, which is about an 10% increase in feed over theprior operation. The internal reflux for both illustrations is also thesame. Internal reflux=Liquid reflux in the column/isobutane product.

The separation was substantially the same in both cases with isobutanepurity in excess of 95 mole %.

                                      TABLE                                       __________________________________________________________________________           STREAM NO.                                                                    12   14    20     28    32         34                                         Descr.                                                                                   Compressed                                                                           Condensed   Tower                                                                              Isobutane                                                                          Internal                              Feed Overhead                                                                            Overhead                                                                             Overhead                                                                            Reflux                                                                              Bottoms                                                                            Product                                                                            Reflux                         __________________________________________________________________________    Prior Operation                                                               Lbs/Hr 306,041                                                                            1,300,000                                                                           1,300,000                                                                            1,300,000                                                                           1,185,000                                                                           --   115,004                                                                            8.5                                   (36,500                 (981,000                                              bbl/day)                Liq)                                                                          (204,000                                                                      Vap)                                           Pressure, Psig                                                                       --   66    139    137   66    --                                       Temp, °F.                                                                     --   92    150    130   130   125                                      Invention                                                                     Lbs/Hr 335,587                                                                            1,300,000                                                                           1,300,000                                                                            1,300,000                                                                           1,174,000                                                                           --   126,107                                                                            8.5                                   (40,000                 (1,075,000                                            bbl/day)                Liq)                                                                          (99,000                                                                       Vap)                                           Pressure, Psig                                                                       --   66    129    127   66    --                                       Temp, °F.                                                                     --   92    144    137   110.7 125                                      __________________________________________________________________________

The invention claimed is:
 1. A process for separating isobutane from amixture of isobutane and n-butane by fractionation comprising:(a)feeding a hydrocarbon stream consisting essentially of isobutane andn-butane to a fractionation zone; (b) supplying heat to saidfractionation zone to vaporize said isobutane and to provide an overheadstream consisting essentially of isobutane vapor; (c) compressing all ofsaid overhead vapor stream to increase the condensing temperaturethereof to a temperature sufficiently greater than the temperature ofthe bottoms of said fractionation zone to provide said heat for saidfractionation; (d) indirectly contacting all of said compressedoverheads with a portion of the bottoms from said fractionation zone toincrease the temperature of said bottoms portion to:(i) vaporize saidportion of the bottoms, (ii) provide boil up in said fractionation zone,and (iii) condense all of said compressed overheads; (e) cooling aportion of the condensed overheads to a temperature no less than thetemperature at the top of said fractionation zone and no greater than20° F. above the temperature at the top of said fractionation zone; and(f) returning said cooled portion of condensed overhead to the top ofsaid fractionation zone as reflux.
 2. The process according to claim 1wherein the pressure in the fractionation zone is in the range of 50 to100 psig.
 3. The process according to claim 2 wherein the temperature atthe top of the fractionation zone is in the range of 90° to 103° F. 4.The process according to claim 3 wherein the temperature in the bottomof the fractionation zone is in the range of 123° to 140° F.
 5. Theprocess according to claim 1 wherein said fractionation zone comprises afractionation column having a plurality of trays and said top of saidfractionation zone comprises the top of said plurality of trays.
 6. Theprocess according to claim 5 wherein the temperature of said cooledportion of condensed overhead is no less than the temperature on the toptray and no greater than 20° F. above the temperature on the top tray.7. The process according to claim 1 wherein said condensed overheads aremaintained at a liquid level during said indirect contacting to extracta maximum amount of heat therefrom by said bottoms portion.
 8. Theprocess according to claim 1 wherein said cooled, condensed andcompressed overheads are passed through an expansion zone to reduce thepressure thereof to that of the fractionation zone.
 9. The processaccording to claim 1 wherein the overheads are compressed to increasethe condensing temperature thereof to a temperarure of 10° to 20° F.above the temperature of the bottoms portion of the fractionation zone.10. The process according to claim 1 wherein the temperature of thereflux is no greater 15° F. above the temperature at the top of saidfractionation zone.
 11. A process for separating isobutane from amixture comprising isobutane and n-butane by fractionationcomprising:(a) feeding a hydrocarbon stream consisting essentially ofisobutane and n-butane to a fractionation zone maintained at a pressurein the range of 50 to 100 psig, a temperature at the top of thefractionation zone in the range of 90° to 103° F. and at the bottom ofthe fractionation zone in the range of 123° to 140° F.; (b) supplyingheat to said fractionation zone comprising a plurality of trays tovaporize said isobutane and to provide an overhead stream consistingessentially of isobutane vapor; (c) compressing all of said overheadvapor stream to increase the condensing temperature thereof to atemperature in the range of 10° to 20° F. greater than the temperatureof the bottoms of said fractionation zone; (d) indirectly contacting allof said compressed overheads with a portion of the bottoms from saidfractionation zone to increase of the temperature of said bottomsportion to:(i) vaporize said portion of the bottoms, (ii) provide boilup in said fractionation zone, and (iii) condense all of said compressedoverhead; (e) cooling a portion of the condensed overhead to atemperature no less than the temperature at the top tray of saidfractionation zone and no greater than 20° F. above the temperature atthe top tray of said fractionation zone; (f) passing said cooledcondensed overhead through an expansion zone, and (g) returning saidcooled portion of condensed overhead to the top of said fractionationzone as reflux.